1. Field of the Invention
The invention relates to devices for providing a predetermined registration of film in a camera, and more specifically, to a device for allowing a partially-exposed 35 mm film strip packaged in a film cassette to be rewound, removed from the camera, subsequently reinserted into the camera, and advanced to the next unexposed frame accurately.
2. Description of the Prior Art
Users of 35 mm cameras have available many different types of film-- for example, monochromatic films of various film speeds and grain characteristics; color films of negative and reversal type; and special process films. It is the practice in the art to package 35 mm film in cartridges or cassettes having sufficient length for 20 or 36 exposures. Both professional and amateur photographers experience occasions in which the film in the camera is not the type appropriate for the particular photographic situation at hand. For such occassions, professional photographers typically utilize multiple cameras, each loaded with a different type of film. Needless to say, this approach is too expensive for many amateur photographers. Large format cameras having interchangeable film-holding backs are also in common use for this purpose, but are not available for 35 mm cameras.
However, when the photographer desires to use one 35 mm camera and this need arises, he may resort to rewinding the partially-exposed film back into its cassette, removing the cassette and installing another cassette of film. To save the unexposed film of the first cassette for future use, he must be very careful not to wind the tongue end of the film completely into the cassette, as it would then not be possible to recover the film end.
When subsequently reinstalling a correctly-rewound cassette into the camera, there is no inherent index on or associated with the film to ensure that the actual positions of the previously exposed frames coincide with the shutter opening or that the numbers of the exposed frames are correctly synchronized with the frame counter of the camera. Consequently, it is common practice to advance the film one or two frames beyond the number of the last exposed frame as read on the film counter. This action creates "safety frames" to prevent possible overlap of the last previously exposed frame and the next frame exposed after such reinstallation. As may be recognized, the safety frame space represents a waste of film. Another problem arises with this method since automatic commercial film processing equipment may cut the film strip into short lengths of four or five frames based on indexing to the first frame. If the space is not an exact integral number of frames, there is the danger that this operation will result in cutting into the picture area of the misaligned, later-exposed frames.